1. Field of the Invention
The invention relates to an optical scanning device for scanning an optical disc record carrier having an information plane and an adjoining layer, the device comprising an objective lens system having a first lens and a second lens, the second lens being arranged between the first lens and the record carrier, for converging a radiation beam through the transparent layer to a focus spot on the information layer.
The dual-lens objective system focusses the radiation beam at a relatively high numerical aperture through an entrance face of the transparent layer. The high numerical aperture causes a small focus spot and allows writing and reading of very high density information on the record carrier. The second lens of the objective system, which is closest to the record carrier, has in general a convex surface facing the first lens and a substantially flat surface facing the record carrier. A focus servo system keeps the focus spot on the information layer during scanning.
2. Description of the Related Art
The optical scanning device disclosed in Japanese patent application no. 8-315404 A uses the focus servo system to control the axial position of the first lens and a further servo system to control the axial position of the second lens. A disadvantage of that device is that its tolerance for tilt of the record carrier decreases rapidly when the thickness of the transparent layer increased.